1. Field of the Invention
The present invention relates to an optical fiber, and in particular to a wide-band dispersion controlled optical fiber which is capable of being used for medium/long distance transmission while using optical signals of wide wavelength band.
2. Description of the Related Art
In an optical communication network for transferring high capacity information in Wavelength Division Multiplexing (WDM) mode, N optical signals with different wavelengths are multiplexed and concurrently transmitted through one strand of an optical fiber. The C-band wavelength region (1530 nm to 1565 nm) and the L-band wavelength region (1570 nm to 1605 nm) are principally used in the optical signals transmitted through the optical fiber, in which the transmission characteristic of optical signals is good. Meanwhile, for the purpose of wide-band/large-capacity transmission in an optical communication network of the WDM mode, researches have been vigorously made in order to use the O-band wavelength region (1285 nm to 1330 nm) and the S-band wavelength region (1460 nm to 1530 nm).
FIG. 1 is a graph showing the dispersion properties of optical fibers according to an example of the prior art. In the graph shown in FIG. 1, a first curve 11 shows a dispersion characteristic of a single mode optical fiber. Second and third curves 13 and 15 show dispersion properties of positive dispersion optical fibers, respectively. A fourth curve 17 shows a dispersion characteristic of a negative dispersion optical fiber. Second and third curves 13 and 15 indicate optical fibers which have positive dispersion values at 1550 nm wavelength, and fourth curve 17 indicates an optical fiber which has a negative dispersion value at 1550 nm wavelength.
Referring to the dispersion characteristic indicated by first curve 11, the C-band wavelength region exhibits dispersion values in the range of 14 to 19 ps/nm/km and the L-band wavelength region exhibits dispersion values larger than those in the C-band wavelength region. Therefore, a dispersion compensator is essentially required in order to employ such a single mode optical fiber in wide-band/large-capacity communication networks, if a single mode optical fiber exhibits the dispersion characteristic as indicated by first curve 11 in WDM communication mode.
In first curve 11, a zero dispersion wavelength is positioned in the O-band wavelength region, which causes deterioration of signal quality due to Four Wave Mixing (FMW) at the time of receiving and sending an optical signal of the O-band wavelength region. The FMW is an example of a non-linear phenomenon produced when sending an optical signal. Due to FMW, two optical signals with different wavelengths are combined, thereby producing one or more new optical signals which distort the assigned optical signals. The FMW is generated at a zero dispersion wavelength at which the phases of optical signals coincide with each other.
The single mode optical fiber with the dispersion characteristic as indicated by first curve 11 has large dispersion values. As a result, the cost is increased for compensating dispersion in WDM mode. Therefore, it is not efficient to construct a wide-band communication network using a single mode optical fiber in a WDM communication network. Accordingly, a single mode optical fiber is mainly used in Time Division Multiplexing (TDM) communication networks for wide-band/large-capacity transmission through the single mode optical fiber.
However, when compared to the TDM communication mode, the WDM communication mode is advantageous in that transmission capacity is greatly increased at small expense. Due to such an advantage, the WDM mode has been continuously developed for optical communication networks.
Because the dispersion values of second and third curves 13 and 15 are distributed lower than those of first curve 11, a cost savings is achieved in that relatively little dispersion compensation is required when constructing a wide-band/large-capacity network of the C-band and the L-band wavelength regions. Second curve 13 exhibits dispersion characteristic of a large effective area optical fiber which has an enlarged effective cross-section area. Third curve 15 indicates a reduced dispersion slope optical fiber which has a reduced dispersion slope.
As shown by second curve 13, a large effective area optical fiber enlarges an effective cross-section area in order to achieve dispersion values which are not less than a predetermined level, thereby avoiding non-linear phenomena such as four wave mixing (FWM). However, because such a large effective area optical fiber has relatively large dispersion values in the L-band wavelength region, dispersion compensation is essentially required. Furthermore, there is a problem in that if the effective cross-section area increases, Raman amplification efficiency decreases.
A reduced slope optical fiber with a dispersion characteristic as indicated by third curve 15 is reduced in difference of dispersion values between the C-band and L-band wavelength regions by reducing the dispersion slope. Therefore, the reduced slope optical fiber reduces the relative difference of dispersion values between the C-band and L-band in a wide-band/large-capacity communication network.
However, because the zero dispersion wavelength of a positive dispersion optical fiber is located in the wavelength region of 1460 nm to 1500 nm as indicated by the second and third curves, it is impossible to use Raman amplification which is used in long distance transmission. Due to the characteristic of the Raman amplification mode, a pumping light which has a wavelength about 100 nm lower than the optical signal that is amplified is used for optical fiber signal amplification. A pumping light of a wavelength region of 1470 nm to 1505 nm is used for amplifying an optical signal of the L-band wavelength region. That is, the wavelengths of pumping light used for amplifying L-band optical signals in the Raman amplification mode are within the wavelength region in which the zero dispersion wavelength of a positive dispersion optical fiber is positioned. Because this induces a non-linear phenomenon such as FWM, it is not efficient to use a positive optical fiber as mentioned above in a wide-band/large-capacity communication network. In addition, a problem exists because the zero dispersion wavelength of a positive dispersion optical fiber is located in the S-band wavelength region of 1460 nm to 1530 nm. Thus, a non-linear phenomenon such as FWM is induced, and it is impossible to use the S-band wavelength region.
A negative dispersion optical fiber with dispersion characteristic as indicated by fourth curve 17 has a zero dispersion wavelength positioned in the wavelength region of 1610 nm to 1700 nm. Therefore, it enables transmission of an optical signal of the C-band and L-band wavelength regions. Such a negative dispersion optical fiber is used only in middle/long distance transmission in the range of about several hundreds km. However, it cannot be used for an optical signal in the S-band or O-band wavelength region because absolute dispersion values are so large.
As described above, despite the fact that wide-band/large-capacity transmission is essentially required since optical communication networks, in particular WDM communication networks have been continuously developed, conventional optical fibers use only optical signals in the C-band wave length region or L-band wavelength region. This is because the positions of zero dispersion wavelengths or dispersion values are not capable of being tuned. Furthermore, a negative optical fiber is useful in middle/long distance transmission networks of about several hundreds km. A positive optical fiber is used in long distance transmission exceeding this range. Consequently, the construction cost of optical communication networks are dually increased, since different types of optical fibers are used in accordance with the transmission length.